In the process of PCB board design, dividing the power plane or ground plane can result in incomplete planes. When signals are routed, the reference plane may transition from one power plane to another, resulting in a phenomenon known as signal cross-segmentation. While cross-division is generally not a concern for low-speed signals, it can have significant effects in high-speed digital signal systems.
High-speed signals rely on the reference plane as the return path. When the reference plane is incomplete, several adverse effects may occur:
1. Discontinuous trace impedance
2. Increased crosstalk between signals
3. Signal reflection
4. Increased current loop area, leading to higher loop inductance and output waveform oscillation
5. Greater radiation interference in space, susceptibility to space magnetic fields
6. Increased risk of magnetic field coupling with other circuits on the board
7. Common mode radiation source and generation through external cables due to high-frequency voltage drop on loop inductance
It is therefore essential to keep PCB board routing as close as possible to a continuous plane and avoid cross-segmentation. This is particularly important for high-speed signal lines, and should only be considered for low-speed signal lines when absolutely necessary.
1. Dealing with Cross-Segmentation in PCB Design
When cross-segmentation inevitably occurs in PCB board design, how should it be handled? In such cases, the segmentation needs to be patched to provide a short return path for the signal. Common processing methods include adding patch capacitors and cross-line bridging.
2. Stitching Capacitors
Typically, a 0402 or 0603 packaged ceramic capacitor is placed at the signal cross section. The capacitance of the capacitor is 0.01uF or 0.1uF. If space allows, several more such capacitors can be added. Additionally, it is important to ensure that the signal line is within the 200mil range of the sewing capacitor, with a smaller distance being preferable. Furthermore, the network at both ends of the capacitor should correspond to the network of the reference plane through which the signal passes. See the network connected at both ends of the capacitor in the figure below. The two colors are highlighted, representing two different networks.
3. Over-the-Line Bridging
For the cross-segmented signal at the signal layer, it is common to “packet the ground,” which may include the signal lines of other networks. This “packet” line should be as thick as possible.
4. Multilayer Wiring
High-speed signal wiring circuits often have high integration and high wiring density. The use of multilayer boards is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of the number of layers can significantly reduce the size of the printed board and make full use of the middle layer to set up shielding, achieve the nearest grounding, reduce parasitic inductance, shorten the transmission length of the signal, and reduce signal cross-interference.
5. Minimize Lead Bends
Lead bends between the pins of high-speed circuit devices should be minimized. The leads of the high-speed signal wiring circuit wiring are all straight lines and need to be turned, which can be done with 45° broken lines or arcs. This requirement is not only for improving the fixing strength of the steel foil in low-frequency circuits but also helps to reduce external emission, mutual coupling of high-speed signals, and the radiation and reflection of signals in high-speed circuits.
6. Minimize Lead Length
The wiring between the pins of the high-speed signal wiring circuit device should be as short as possible. Longer leads result in greater distributed inductance and capacitance, which can significantly influence the passage of high-frequency signals and change the characteristic impedance of the circuit, leading to reflections and oscillations.
7. Minimize Alternating between Lead Layers
The fewer vias used in the component connection process, the better. According to measurements, one via can bring about 0.5pF of distributed capacitance, resulting in a significant increase in circuit delay. Reducing the number of vias can significantly improve speed.
8. Beware of Parallel Cross-Interference
In high-speed signal wiring, attention should be paid to the “cross-interference” introduced by parallel wiring of signal lines in close proximity. If parallel distribution cannot be avoided, a large area of “ground” can be arranged on the opposite side of the parallel signal lines to greatly reduce interference.
9. Avoid Branches and Stubs
High-speed signal routing should aim to avoid branches and stubs, which have a significant impact on impedance and can cause signal reflections and overshoots. Using daisy-chain wiring can reduce the impact on the signal.
10. Inner Layer Routing for Signal Lines
High-frequency signal lines are prone to generate large electromagnetic radiation on the surface layer and are susceptible to interference from external electromagnetic radiation. Routing the high-frequency signal line between the power supply and the ground line can greatly reduce the radiation by absorbing electromagnetic waves on the PCB board.
High-speed signals rely on the reference plane as the return path. When the reference plane is incomplete, several adverse effects may occur:
1. Discontinuous trace impedance
2. Increased crosstalk between signals
3. Signal reflection
4. Increased current loop area, leading to higher loop inductance and output waveform oscillation
5. Greater radiation interference in space, susceptibility to space magnetic fields
6. Increased risk of magnetic field coupling with other circuits on the board
7. Common mode radiation source and generation through external cables due to high-frequency voltage drop on loop inductance
It is therefore essential to keep PCB board routing as close as possible to a continuous plane and avoid cross-segmentation. This is particularly important for high-speed signal lines, and should only be considered for low-speed signal lines when absolutely necessary.
1. Dealing with Cross-Segmentation in PCB Design
When cross-segmentation inevitably occurs in PCB board design, how should it be handled? In such cases, the segmentation needs to be patched to provide a short return path for the signal. Common processing methods include adding patch capacitors and cross-line bridging.
2. Stitching Capacitors
Typically, a 0402 or 0603 packaged ceramic capacitor is placed at the signal cross section. The capacitance of the capacitor is 0.01uF or 0.1uF. If space allows, several more such capacitors can be added. Additionally, it is important to ensure that the signal line is within the 200mil range of the sewing capacitor, with a smaller distance being preferable. Furthermore, the network at both ends of the capacitor should correspond to the network of the reference plane through which the signal passes. See the network connected at both ends of the capacitor in the figure below. The two colors are highlighted, representing two different networks.
3. Over-the-Line Bridging
For the cross-segmented signal at the signal layer, it is common to “packet the ground,” which may include the signal lines of other networks. This “packet” line should be as thick as possible.
4. Multilayer Wiring
High-speed signal wiring circuits often have high integration and high wiring density. The use of multilayer boards is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of the number of layers can significantly reduce the size of the printed board and make full use of the middle layer to set up shielding, achieve the nearest grounding, reduce parasitic inductance, shorten the transmission length of the signal, and reduce signal cross-interference.
5. Minimize Lead Bends
Lead bends between the pins of high-speed circuit devices should be minimized. The leads of the high-speed signal wiring circuit wiring are all straight lines and need to be turned, which can be done with 45° broken lines or arcs. This requirement is not only for improving the fixing strength of the steel foil in low-frequency circuits but also helps to reduce external emission, mutual coupling of high-speed signals, and the radiation and reflection of signals in high-speed circuits.
6. Minimize Lead Length
The wiring between the pins of the high-speed signal wiring circuit device should be as short as possible. Longer leads result in greater distributed inductance and capacitance, which can significantly influence the passage of high-frequency signals and change the characteristic impedance of the circuit, leading to reflections and oscillations.
7. Minimize Alternating between Lead Layers
The fewer vias used in the component connection process, the better. According to measurements, one via can bring about 0.5pF of distributed capacitance, resulting in a significant increase in circuit delay. Reducing the number of vias can significantly improve speed.
8. Beware of Parallel Cross-Interference
In high-speed signal wiring, attention should be paid to the “cross-interference” introduced by parallel wiring of signal lines in close proximity. If parallel distribution cannot be avoided, a large area of “ground” can be arranged on the opposite side of the parallel signal lines to greatly reduce interference.
9. Avoid Branches and Stubs
High-speed signal routing should aim to avoid branches and stubs, which have a significant impact on impedance and can cause signal reflections and overshoots. Using daisy-chain wiring can reduce the impact on the signal.
10. Inner Layer Routing for Signal Lines
High-frequency signal lines are prone to generate large electromagnetic radiation on the surface layer and are susceptible to interference from external electromagnetic radiation. Routing the high-frequency signal line between the power supply and the ground line can greatly reduce the radiation by absorbing electromagnetic waves on the PCB board.
